Heterogeneous Orientational Relaxations and Translation-Rotation Decoupling in (Choline Chloride + Urea) Deep Eutectic Solvents: Investigation through Molecular Dynamics Simulations and Dielectric Relaxation Measurements

Molecular dynamics simulations and dielectric relaxation (DR) measurements in the frequency window, 0.2 ≤ ν/GHz ≤ 50, have been performed to explore the heterogeneous reorientation dynamics in [f choline chloride + (1 - f) urea] deep eutectic solvents (DESs) at f = 0.33 and 0.40 in the temperature range 293 ≤ T/K ≤ 333. The solution viscosity varies by more than an order of magnitude. DR measurements in these DESs reveal multiple relaxation timescales-τ₁ ∼ 500 ps, τ₂ ∼ 100 ps, τ₃ ∼ 30 ps, and τ₄ ∼ 5 ps. Simulated rank-dependent collective single-particle reorientational (C(t), l being the rank) and structural H-bond [C_HB(t)] relaxations can explain the microscopic origin of all these DR timescales. The average DR times, ⟨τ_DR⟩, exhibit a pronounced fractional viscosity dependence, ⟨τ_DR⟩ ∝ (η/T), with p = 0.1. This experimental evidence of pronounced heterogeneous reorientation dynamics in these DESs is supported by a strong translation-rotation decoupling and a significant deviation of the average reorientational correlation times (⟨τ⟩) from Debye's l(l + 1) law. The simulated ratios between the average rotation and translation timescales for both urea and choline correctly reduce to the appropriate hydrodynamic limit at high temperatures. The stretched exponential relaxations of the simulated self-dynamic structure factors and the non-Gaussian single-particle displacement distributions further support strong temporal heterogeneity in these DESs. Dynamic susceptibilities from the simulated four-point correlations exhibit long correlated timescales. Moreover, simulated activation energies estimated from the temperature-dependent C₁(t) decays and the translational diffusion coefficients from the velocity autocorrelation functions agree favorably with those from the corresponding DR and the pulsed field gradient nuclear magnetic resonance measurements.